1. Field of the Invention
The present invention relates to a process for producing lower olefins (light olefins) from a mixture containing dimethyl ether and methanol. More particularly, the invention relates to a process for producing lower olefins from a mixture containing carbon monoxide and hydrogen and further containing dimethyl ether and methanol in a specific ratio, with high energy efficiency.
2. Description of the Prior Art
In recent years, a demand for propylene has been greater in the market than that for ethylene, and hence the development of a process for producing lower olefins in a high yield of propylene at a low production cost has been desired.
Lower olefins such as ethylene and propylene are industrially produced mainly by thermal cracking using ethane or naphtha as a starting material. In the process for producing lower olefins by an ethane cracker using ethane as a starting material, a major component of the resulting lower olefins is ethylene and the content of propylene is very low, so that production of propylene by this process is not practical.
In the lower olefins obtained by a naphtha cracker using naphtha as a starting material, the propylene content is higher than that in the lower olefins produced by the ethane cracker, but they still do not agree with the demand-supply balance of ethylene/propylene in the market.
Further, production of lower olefins by the ethane cracker or the naphtha cracker has a problem of high energy cost because high-temperature thermal cracking at about 800 to 1,000° C. is generally carried out.
Moreover, production of lower olefins using a petroleum type starting material such as naphtha is not economical because the material is expensive. In addition, by-products such as methane and hydrogen are formed in large amounts, and separation of the by-products needs great energy. Therefore, a process for producing lower olefins such as ethylene and propylene from a starting material other than the petroleum type starting material such as naphtha has been desired.
The process using no petroleum type starting material is, for example, a process for producing lower olefins from methanol. In this process, generally, a mixture containing methanol (crude methanol) is formed from a synthesis gas, then high-purity methanol is isolated from the mixture by distillation or the like, and the resulting methanol is allowed to undergo reaction at an intermediate temperature such as a temperature of about 300 to 600° C. to produce lower olefins. According to this process, lower olefins can be obtained in a higher yield of propylene than that in the process using the ethane cracker or the naphtha cracker, and the ratio between ethylene and propylene can be flexibly controlled.
In such a conventional process for producing lower olefins from methanol as described above, however, methanol having higher purity than the mixture is isolated and used, so that there resides a problem of high equipment cost and high energy cost related to the isolation of methanol.
Under the above circumstances, there has been proposed a process for producing lower olefins in which crude methanol is formed from a synthesis gas and this crude methanol is used without being purified (U.S. Pat. No. 5,714,662). According to this process, a mixture containing methanol and a small amount of dimethyl ether (sometimes referred to as “DME” hereinafter) produced as a by-product is formed from a synthesis gas, and this mixture is used as a starting material for the olefin production to produce C2-C4 olefins. In this process, further, the resulting butene fraction is converted into an ether of high octane value and used.
Under such circumstances as mentioned above, the present inventors have earnestly studied a process for producing lower olefins more efficiently, and as a result, they have found that lower olefins containing ethylene and propylene in a desired ratio can be produced with particularly high energy efficiency by converting a mixed fluid containing DME and methanol in a specific weight ratio into lower olefins. Based on the finding, the present invention has been accomplished.
It is an object of the present invention to provide a process for producing lower olefins, by which lower olefins containing ethylene and propylene can be produced from a mixed fluid containing carbon monoxide and hydrogen and further containing DME and methanol with high energy efficiency.